Angiotensin II, the substance we mentioned earlier, is a chemical that binds to receptors on blood vessels, causing them to contract. Often, doctors prescribe ACE inhibitors to prevent the formation of the enzyme (angiotensin I) that's responsible for producing angiotensin II. But ACE inhibitors don't work for everyone, and some patients have trouble with the side effects (which can include kidney failure). In such cases, doctors may opt to prescribe angiotensin II receptor blockers, or ARBs.
While ACE inhibitors prevent angiotensin II from forming, ARBs prevent it from working. How? ARBs stop angiotensin II from reaching its specialized receptors, located on the exterior of blood vessels. This keeps them floating nearby, where they can't do any harm.